Production of nitrogen free of light impurities

Abstract

This invention relates to a cryogenic process for the separation of air utilizing an integrated multi-column distillation system wherein an ultra high purity nitrogen rich, oxygen rich and optionally an argon rich product are generated. In the cryogenic distillation separation of air, air is initially compressed, pretreated and cooled for separation into its components. Ultra high purity, e.g., nitrogen having less than 0.1 ppm of volatile impurities is generated with enhanced nitrogen product recovery by withdrawing liquid nitrogen from the higher pressure column at an intermediate point and charging that fraction as reflux to the lower pressure column, withdrawing a nitrogen stream which is rich in volatile contaminants from the top of the high pressure column, partially condensing the nitrogen stream and removing the uncondensed portion as a purge stream, and, withdrawing an ultra high purity nitrogen product at a point below a nitrogen vapor withdrawal point at the top of the lower pressure column. Alternatively, no purge need be taken and the volatile impurities allowed to pass to the low pressure column. In that case, a nitrogen fraction rich in volatile impurities is removed from an upper portion of the low pressure column, condensed and at least a portion of the uncondensed fraction removed as a purge and the condensed portion returned to the low pressure column.

What is claimed is: 
     
       1. In a process for the cryogenic separation of air which comprises nitrogen, oxygen and volatile impurities in an integrated multi-column distillation system, having a higher pressure column and a lower pressure column wherein the air stream is compressed, freed of condensible impurities, and cooled generating a feed for the integrated multi-column distillation system, the improvement for producing ultra high purity nitrogen at high nitrogen recovery which comprises: a) generating a liquid nitrogen fraction and a nitrogen rich vapor fraction containing volatile impurities near the top of the higher pressure column;   b) removing a portion of the liquid nitrogen fraction from the higher pressure column;   c) expanding the liquid nitrogen fraction and introducing the expanded fraction to the top of the lower pressure column as feed;   d) generating a nitrogen rich vapor fraction containing residual volatile impurities at the top of the lower pressure column and removing that fraction as an overhead;   e) partially condensing at least one of said nitrogen rich vapor fractions generated in step (a) or (d) or both in a boiler/condenser;   f) removing at least a portion of at least one of the uncondensed nitrogen rich vapor fractions concentrated in volatile impurities from the boiler/condenser as a purge stream;   g) returning at least a portion of at least one of the condensed nitrogen rich vapor fractions to a column as reflux; and,   h) generating and removing an ultra high purity nitrogen fraction as product from the lower pressure column at a point below the removal point for the nitrogen rich vapor containing volatile impurities and below the point of return of the liquid nitrogen fraction as reflux to the lower pressure column.   
     
     
       2. The process of claim 1 wherein the liquid nitrogen from the higher pressure column is expanded and the volatile impurities flashed therefrom in a separator. 
     
     
       3. The process of claim 2 wherein at least a portion of the nitrogen liquid obtained from the separator is returned to an upper portion of the lower pressure column as nitrogen reflux. 
     
     
       4. The process of claim 3 wherein a liquid nitrogen product is withdrawn from the lower pressure column at a point about 2-5 trays below the removal point for the nitrogen vapor containing residual volatile impurities. 
     
     
       5. The process of claim 3 wherein a nitrogen vapor product stream is withdrawn from an upper portion of the lower pressure column at a point below the removal point for nitrogen-rich vapor containing residual impurities. 
     
     
       6. The process of claim 2 wherein liquid nitrogen from the high pressure column is charged at the top of a distillation column and volatile components stripped therefrom with the resulting liquid fraction being charged to the separator used for introducing liquid nitrogen to the lower pressure column as reflux. 
     
     
       7. The process of claim 6 wherein the separator for returning liquid nitrogen to the lower pressure column is a distillation column. 
     
     
       8. The process of claim 7 wherein a nitrogen fraction rich in volatile impurities is generated in the lower pressure column and a portion of the nitrogen rich vapor fraction containing volatile impurities from the lower pressure column is charged to a separate boiler/condenser and condensed with the condensed fraction returned as reflux to the lower pressure column and the uncondensed fraction removed as a purge. 
     
     
       9. In a process for the cryogenic separation of an air stream in an integrated multi-column distillation system having a higher pressure column, a lower pressure column, and a side arm column for separation of argon, the improvement for producing ultra high purity nitrogen product, while enhancing nitrogen recovery which comprises: a) feeding substantially all of said cooled air stream to the higher pressure column;   b) generating a liquid nitrogen fraction and a nitrogen rich vapor fraction containing volatile impurities near the top of the higher pressure column;   c) removing a portion of the liquid nitrogen fraction from the higher pressure column at a point below a removal point designated for the removal of a nitrogen rich vapor fraction containing volatile impurities;   d) expanding the liquid nitrogen fraction and introducing the expanded fraction to the top of the lower pressure column as feed;   e) generating a nitrogen rich vapor fraction containing residual volatile impurities fraction at the top of the lower pressure column and removing that fraction as an overhead; and,   f) partially condensing at least one of said nitrogen rich vapor fractions generated in step (b) or step (e) in a boiler/condenser and returning at least a portion of at least one the condensed nitrogen rich vapor fractions to a column as reflux;   g) removing at least a portion of at least one of the uncondensed nitrogen rich vapor fractions concentrated in volatile impurities generated in step (f) from the boiler/condenser as a purge stream;   h) removing an argon stream from the low pressure column and fractionating that argon stream in said side arm column and recovering an argon rich product as overhead; and,   j) generating and removing an ultra high purity nitrogen fraction as product from the lower pressure column at a point below the removal point for the nitrogen rich vapor containing residual volatile impurities and below the point of return of the liquid nitrogen fraction as reflux to the lower pressure column.   
     
     
       10. The process of claim 9 wherein a nitrogen fraction rich in volatile impurities is generated in the high pressure column and a portion of the nitrogen rich vapor fraction containing volatile impurities from the high pressure column is charged to a separate boiler/condenser and condensed with the condensed fraction returned as reflux to the high pressure column and the uncondensed fraction removed as a purge. 
     
     
       11. The process of claim 10 wherein the liquid nitrogen from the higher pressure column is expanded and the volatile impurities flashed therefrom in a separator. 
     
     
       12. The process of claim 11 wherein at least a portion of the nitrogen liquid obtained from the separator is returned to an upper portion of the lower pressure column as nitrogen reflux. 
     
     
       13. The process of claim 12 wherein liquid nitrogen product is withdrawn from the lower pressure column at a point about 2-5 trays below the removal point for the nitrogen vapor containing residual volatile impurities. 
     
     
       14. The process of claim 13 wherein a nitrogen vapor product stream is withdrawn from an upper portion of the lower pressure column as a point below the removal point for nitrogen-rich vapor containing residual impurities. 
     
     
       15. The process of claim 14 wherein liquid nitrogen from the high pressure column is charged at the top of a distillation column and volatile components stripped therefrom with the resulting liquid fraction being charged to the separator used for introducing liquid nitrogen to the lower pressure column as reflux. 
     
     
       16. The process of claim 14 wherein the separator for returning liquid nitrogen to the lower pressure column is a distillation column. 
     
     
       17. The process of claim 16 wherein liquid oxygen is withdrawn from the bottom of the lower pressure column, expanded, separated into liquid and vapor components in a separator and the liquid component vaporized in a boiler/condenser in the argon column. 
     
     
       18. The process of claim 17 wherein the separator for separating the expanded liquid oxygen is a distillation column and the liquid oxygen is cooled in a boiler/condenser with said distillation prior to expansion and the resulting expanded oxygen charged to the top of said distillation column.